New transistor device architectures such as double gate transistor devices have been developed to provide improved short channel control which results in improved current characteristics. A FinFET device is an example of such a double gate device. A simplified cross-section of part of a FinFET device is shown in FIG. 1. A FinFET device is a non-planar device which includes a conducting channel that is formed on the two opposite faces of or is wrapped around a thin silicon ‘fin’ 14 formed on a substrate 12 which forms the body of the device. A gate 18 overlies the fin 14 and a gate dielectric 16. The dimensions of the fin and the number of fins in the device determine the effective channel width of the device. The FinFET has some limitations due to the space between the fins being electrically inactive.
An ITFET device has been developed which is a FinFET-type device with lateral extensions at the bottom of the fin so as to provide active thin body devices in the unused regions between the fins which increases the current drive compared to a FinFET device. A simplified cross-section of part of an ITFET device is shown in FIG. 2. The fin 24 and lateral extensions 25 form an inverted T shaped channel structure providing horizontal and vertical channels which are controlled by multiple contiguous gate segments: gate 28 overlies the fin 24 and a gate dielectric 26. Thus, an ITFET device is a multi gate device combining vertical and planar thin body structures within a single device. More details of the ITFET device can be found in US patent application no. 2007/050317 and an article entitled ‘Inverted T channel FET (ITFET)—Fabrication and Characteristics of Vertical-Horizontal, Thin Body, Multi-Gate, Multi-Orientation Devices, ITFET SRAM Bit-cell operation. A Novel Technology for 45 nm and Beyond CMOS’, by L. Mathew et al, Electron Devices Meeting, 2005. IEDM Technical Digest. IEEE International 5-7 Dec. 2005, pages 713-716.
In order not to compromise device performance, the inverted T shaped channel structure needs to be patterned reliably and repeatedly which is a challenge particularly since the structure has eight corners. Currently, as discussed in the above two documents, the lateral extensions of the inverted T shaped channel structure of the ITFET device is manufactured by a timed etch. However, generally, a timed etch process (i.e. a main etch process that is performed for a predetermined time) does not offer much control and uniformity in a manufacturing environment. In other words, with the current process, the thickness of the horizontal channel provided by the lateral extensions is hard to control reliably and uniformly.
Thus, there is therefore a need for an improved method of forming an ITFET device.